The present invention relates to a stage structure of an axial flow turbine where fluid flows in the axial direction. More particularly, the present invention relates to a stage structure of an axial flow turbine that can reduce the stage loss arising from turbine stages.
A typical conventional stage structure of an axial flow turbine will be described below for its configuration by referring to FIG. 5.
FIG. 5 is a schematic illustration of a structure of stages of an axial flow turbine where fluid flows in the axial direction. A plurality of stationary blades 3 are arranged in a row at predetermined regular intervals in a peripheral direction between a diaphragm outer ring 1 and a diaphragm inner ring 2. A plurality of moving blades 6 are arranged facing the turbine stationary blades at the downstream side of the turbine stationary blades. The turbine moving blades 6 are arranged respectively at the outer peripheries of rotor disks 4 in a row at predetermined regular intervals in a peripheral direction.
The turbine stationary blades 3 arranged in the above-described manner lead the main stream 7 of turbine working fluid between the blades and allow it to pass through them so as to make the stationary blade inlet pressure P1 decrease the outlet pressure P2 and accelerate the move of working fluid. The fluid, which has passed turbine stationary blades 3 and has been accelerated by the latter, then flows to the moving blades 6, and the kinetic energy of the fluid is converted into rotational mechanical energy.
On the other hand, the stationary blades 3 and the moving blades 6 resist the flow of fluid so that the flowing site of fluid that passes the stationary blades 3 and the moving blades 6 gives rise to a turbulence loss. Major losses that arise in the row of blades include a blade element loss (hereinafter referred to as “profile loss”) and a secondary loss which takes place on the wall surfaces of the root sections and the tip sections of the blades of the row. Besides the profile loss and the secondary loss listed above, losses arise from between adjacent stages includes: a shaft leak loss produced by fluid flowing through the gap (or the labyrinth) between each stationary blade diaphragm inner ring 2 and the rotor shaft; a blade tip leak loss produced by fluid flowing through the gap (or the labyrinth) between the tip of each moving blade 6 and the corresponding stationary blade diaphragm outer ring 1; and a moisture loss.
FIG. 6 shows a typical breakdown of the loss that arises between stages. The leak losses that take place at the shaft and at the blade tip 3 are unignorably large if compared with the profile loss and the secondary loss that are recognized as major losses in the row of blades of the stages. Particularly, since the blade tip leaking fluid 10 does not pass the row of blades and hence does not work in the stages, the quantity of the leaking fluid directly affects the loss of the entire stages. The magnitude of the leak loss in each stage is determined as a function of: the distance of the gap between the stationary blade diaphragm inner ring and the shaft or the distance of the gap (labyrinth) between the shroud 5 of the moving blade tip and the corresponding stationary blade diaphragm outer ring 1, and the pressure difference between the stationary blade and the moving blade. Therefore, the leak loss can be theoretically reduced by reducing the gap between the stationary blade diaphragm inner ring 2 and the shaft or the distance of the gap (labyrinth) between the shroud 5 of the moving blade tip and the corresponding stationary blade diaphragm outer ring 1. However, the gaps cannot be reduced less than a certain lower limit, because the influence of the elongation of the rotor and that of the diaphragms by heat needs to be taken into consideration for actual operations.
Now, the general flow of fluid between stages in an axial flow turbine will be described below by referring to FIG. 5. When the main flow 7 of fluid enters the nozzle, part of the fluid of the main flow 7 passes through the gap between the stationary blade diaphragm inner ring 2 and the shaft as shaft leaking fluid 8 and then joins the main flow at the moving blade inlet.
Similarly, when the main flow 7 of fluid passes the moving blade 6, part of the fluid of the main flow 7 passes through the gap between the moving blade tip shroud 5 and the stationary blade diaphragm outer ring 1 as blade tip leaking fluid 10 and then joins the main flow at the stationary blade inlet of the next stage. A local turbulence occurs in the main flow on and near the wall surface where such leaking fluid departs from and joins the main flow. Then, the angle of flow is shifted on and near the wall surface due to the local turbulence of the main flow to increase the difference between the geometrical angle of the blade front edge and the angle of the main flow to increase the loss (incidence loss) in the row of blades. Additionally, the turbulence of the main flow promotes the development of the secondary flow vortexes, which arise at the roots of the moving blades and at the tips of the stationary blades, at the moving blade inlet and the stationary blade inlet where leaking fluid joins the main flow. In this way, the stage loss is increased as leaking fluid interferes with the main flow and the influence of the stage loss becomes more significant as the quantity of leaking fluid increases.
In view of the above-identified problems, various methods have been proposed to date for the purpose of reducing the quantity of leaking fluid and the interference of leaking fluid relative to the main flow.
Currently, a technique of arranging a plurality of fins 11 in the gap between the stationary blade diaphragm inner ring 2 and the shaft or in the gap between the moving blade tip and the stationary blade diaphragm outer ring 1 in order to reduce leaking fluid as shown in FIG. 5 is known as a technique for reducing the leak losses at the shaft and at the blade tips. Japanese Patent Application Laid-Open Publication No. 2006-097544 discloses a technique of arranging a labyrinth seal that is formed by a plurality of fins in the gap between the stationary blade diaphragm inner ring and the shaft and also in the gap between the moving blade tips and the stationary blade diaphragm outer ring.
An improved efficiency of turbines can save energy and hence is believed to contribute to the policy of reducing the environment load. Efforts are currently being paid to develop high efficiency turbines.
Particularly, the efficiency of turbines is improved by improving the performance of turbine stages. In other words, the efficiency of turbines is improved effectively by reducing the losses that arise in turbine stages and thereby improving the performance of turbines. Particularly, the performance of turbine stages can be improved remarkably by reducing leak losses and blade row losses that arise when leaking fluid interferes with the main flow. As pointed out above, techniques for reducing the leak loss at the shaft and the blade tips have been proposed. Such proposed techniques include those for reducing leaking fluid by arranging several fins in the gap between the stationary blade diaphragm inner ring and the shaft or the gap between the moving blade tips and the stationary blade diaphragm outer ring.
However, no technique for suppressing the interference of leaking fluid with the main flow after passing the gap between the moving blade tips and the stationary blade diaphragm outer ring has been established to date.